Es. Krull et al., delta C-13(org) chemostratigraphy of the Permian-Triassic boundary in the Maitai Group, New Zealand: evidence for high-latitudinal methane release, NZ J GEOL, 43(1), 2000, pp. 21-32
Carbon isotopic studies from marine organic matter of the Permian-Triassic
Maitai Group, New Zealand, reveal a significant delta(13)C(org) shift towar
d more negative values within the Little Ben Sandstone Formation. These iso
topic data chemostratigraphically define the previously debated position of
the Permian-Triassic boundary in the Maitai Group. The Permian-Triassic re
cord of the Maitai Group is also important because of its high paleolatitud
inal setting and the deposition at intermediate depths in the ocean (c. 400
m) within a volcanic arc-related basin. Marine Permian-Triassic strata dep
osited at water depths deeper than shelf areas are rare. High latitude Perm
ian-Triassic boundary sections document a significantly larger isotopic off
set across the boundary compared with lower latitude settings.
Carbon isotopic values decrease rapidly by an average of 7 parts per thousa
nd from homogeneous values ((x) over bar -25 parts per thousand) in the Tra
mway and lower Little Ben Sandstone Formation to highly fluctuating and ver
y depleted values ((x) over bar -32 parts per thousand) within the Little B
en Sandstone Formation. The lowermost Big Ben and Tramway Formations are co
nsidered to be Permian in age, based on their homogeneous and comparably he
avier carbon isotopic values and supported by fossil atomo-desmatinid bival
ves. Based on the distinct delta(13)C(org) excursion toward negative values
and the concurrent onset of strong isotopic fluctuations, the Permian-Tria
ssic boundary is placed in the lower half of the Little Ben Sandstone Forma
tion.
Very depleted delta(13)C values in the Little Ben Sandstone Formation of-38
parts per thousand indicate a contribution from isotopically light methane
. A possible methane source is clathrates, released by large submarine slid
es or warming-induced melting of permafrost. The Little Ben Sandstone Forma
tion has been interpreted as a massive event deposit from a submarine slide
(Landis 1980). This hypothesised methane release could have been in part r
esponsible for the larger Permian-Triassic isotopic shift in high latitudes
compared with low latitudes because large volumes of clathrates are trappe
d in continental shelves and high-latitude permafrost.